The economic production of argon via air separation plants is linked to the production of equivalent quantities of nitrogen or oxygen or both. In recent years, the demand for argon has been growing at a more rapid rate than the corresponding growth rate of either nitrogen or oxygen. Alternative sources for argon has thus become very attractive. One such alternative source is the purge gas from an ammonia synthesis plant.
In an ammonia synthesis plant, it becomes necessary to purge a fraction of the gas stream in order to maintain the inert concentration below a specified level. Undesirably high inert levels reduce the partial pressure of the reactants and cause an unfavorable shift of the ammonia synthesis reaction equilibrium. Methane and argon are the inert gases of concern. A typical composition of the ammonia purge gas, available at approximately 1900 psig, is as follows: 60.5% H.sub.2, 20% N.sub.2, 4.5% Ar, 13% CH.sub.4 and 2% NH.sub.3.
Since the ammonia synthesis process is energy intensive, economics have favored recovery of the hydrogen in the purge gas for recycle to the ammonia synthesis loop. Currently, three kinds of processes are employed for this purpose; in order of most to least prevalent, these are membrane separation, cryogenic separation and pressure swing adsorption separation. In fact, the use of a membrane separator has become very popular for hydrogen recovery and recycle, and a number of ammonia plants in the United States and abroad have hydrogen membrane separator units installed.
Present technology for argon recovery from ammonia synthesis plant purge gas does not optimally integrate this hydrogen membrane separator, but rather employs a cryogenic process that consists of a pre-treatment section for ammonia removal and three subsequent separatory columns. In such a conventional design, the first two columns are for stripping hydrogen and nitrogen in the feed gas and the final column is for separating argon and methane to obtain pure liquid argon product and also pure methane for use as fuel.
The primary object of the invention was to develop an improved process for the recovery of argon from ammonia plant purge gas. A further object of the present invention was to develop a process employing an advantageous combination of non-cryogenic and cryogenic separatory steps for post membrane recovery of argon from the non-permeate gas stream. Yet a further object of the present invention was to develop a PSA system to accomplish removal of methane in the purge gas exiting an ammonia synthesis plant.
In the following description of the invention, the term "pressure swing adsorption" or its acronym "PSA" is used in reference to a type of process and apparatus that is now well known and widely used with respect to separating the components of a gaseous mixture. A PSA system basically comprises passing a feed gas mixture through one or more adsorption beds containing a sieve material which has a greater selectivity for a more strongly adsorbed component than a more weakly adsorbed component of the gas mixture. In the operation of a typical 2-bed PSA system, the connecting conduits, valves, timers, and the like are coordinated and arranged so that when adsorption is occurring in a first bed, regeneration is occurring in a second bed. In the usual cycle, sequential steps with respect to each bed include bed pressurization, product release and venting. Basic PSA systems are described in U.S. Pat. No. 2,944,627, U.S. Pat. No. 3,801,513, and U.S. Pat. No. 3,960,522.
Various modifications and improvements to the basic PSA process and apparatus have been described in the literature, for example, in U.S. Pat. No. 4,415,340, issued on Nov. 15, 1983 and U.S. Pat. No. 4,340,398 issued on July 20, 1982.
The present invention is not limited to the use of any particular PSA process or apparatus design. A specific design that leads to high argon yield is, however, detailed below as an example.